1. Field of the Invention
The present invention relates to an optical fiber, and more particularly to a device for monitoring a coating concentricity of an optical fiber and an optical fiber suitable for the same device.
2. Description of the Related Art
In general, an optical fiber comprises a core as a light transmission medium, a clad surrounding the core, a first coating layer which surrounds the clad and is formed of a soft material having a relatively low modulus, and a second coating layer which surrounds the first coating layer and has a relatively high modulus. In other words, the optical fiber has a double-coating structure. A conventional fiber-drawing tower has a coating unit for forming the first and the second coating layers, a curing unit for curing the first and the second coating layers, and a coating concentricity monitor disposed between the coating unit and the curing unit. The coating concentricity monitor is used for monitoring optical fiber vibration and a coating concentricity error that can be generated while the optical fiber passes through the coating unit. The coating concentricity monitor irradiates a light to a double-coated optical fiber in order to obtain a pattern that appears on a light-receiving surface after the light has passed through the optical fiber. It is possible to know the coating concentricity of the double-coated optical fiber from the said pattern. Since an optical fiber preform tends to become larger in its diameter through technological developments in optical fiber production processes, a high-speed fiber-drawing process becomes more important for the purpose of reducing the manufacturing cost and increasing the unit productivity.
However, the more the fiber-drawing speed increases, the more the temperature of the fiber increases, and the temperature increased thereby causes a variation in the refraction index values of the first and the second coating layers of the optical fiber. As the fiber-drawing speed increases, it is possible to monitor an optical fiber vibration and a coating concentricity error within a certain range of refraction index variations. However, it is not possible to monitor them above the range, because an overlapping of the pattern appearing on the light-receiving surface is generated by the increase of the refraction angle of the light for monitoring.
It is accordingly desirable to provide a device for monitoring a coating concentricity in a high-speed fiber-drawing process.